There is known at present a respiratory apparatus with a semi-closed circuit in which the diver is connected to a small deformable bag or container at the same pressure as the exterior and the diver breathes a respiratory mixture composed of oxygen and a neutral gas, generally nitrogen or helium. This small bag communicates, through a set of valves, with a large deformable bag or container when the diver breathes in and with the exterior when he breaths out. The large container communicates with bottles of pure oxygen and neutral gas, and the percentage, by volume, of oxygen in the large container is automatically regulated by mechanical means.
Such apparatus is not suitable for diving to great depths beyond 200 meters, as the percentage, by volume of the oxygen necessary at these depths is less than 3% and the preparation of the respiratory mixture becomes very delicate.
Another disadvantage of such apparatus arises from the fact that the proportion of neutral gas in the mixture becomes increasingly greater with the depth. Since some neutral gas is discharged to the exterior at the time of each expiration by the diver, the consumption of neutral gas is substantial, which limits the range of the apparatus. For military applications, notably for combat divers, the apparatus with semi-closed circuit also presents the disadvantage of constantly emitting bubbles which permit locating the divers.
There is also known respiratory apparatus for divers having a closed circuit comprising a deformable respiratory bag or container in pressure equilibrium with the exterior which communicates with a cartridge containing an absorbant or CO.sub.2. The diver aspirates or expirates into the bag such that the neutral gas is constantly recirculated. The apparatus comprises a bottle of pure oxygen and regulation means for maintaining a determined proportion of oxygen in the respiratory bag.
The apparatus of this type does not consume neutral gas and the diver is not locatable from the surface. Such apparatus permits diving to depths of about 30 meters.